Implant

ABSTRACT

An implant ( 1 ) for insertion into and permanent anchorage in bone tissue having an intraosseous anchoring structure ( 3 ). The anchoring structure ( 3 ) has a generally circular cross-section and comprises a first cylindrical section ( 11 ) of a first diameter, a second cylindrical section ( 13 ) of a second diameter less than the first diameter, said first and second cylindrical sections ( 11, 13 ) each being provided with a screw thread profile. The anchoring structure ( 3 ) also comprises a tapered connecting section ( 15 ) provided between an interconnecting said first and second cylindrical sections ( 11, 13 ).

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SE00/01944 which has an Internationalfiling date of Oct. 6, 2000, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The present invention relates to an implant for insertion into andpermanent anchorage in human skeletal or bone tissue. More specifically,the invention concerns an implant having a screw-threaded anchoringstructure comprising a first cylindrical section of a first diameter anda second cylindrical section of a second diameter, said second diameterbeing less than said first diameter.

The invention is particularly, although not exclusively, concerned withan implant of this type which takes the form of a femur fixture for ahip-joint pros* thesis.

BACKGROUND OF THE INVENTION

Implants of this type in the form of femur fixtures for a hip-jointprosthesis are known from Applicant's prior International patentapplication publications WO93/01769, WO93/16663 and WO97/25939 with thefirst and second cylindrical sections being provided with external screwthreads for engaging the bone tissue of the femur. The screw threadedfirst and second cylindrical sections of the femur fixture-disclosed inWO93/01769 are assembled together in the femur by firstly inserting thesecond cylindrical section medially into the neck of the femur frombeneath the greater trochanter and then inserting the first cylindricalsection into the neck laterally through the resected section left afterresection of the head of the femur. The screw threaded first and secondcylindrical sections of the femur fixture disclosed in WO93/16663 andWO97/25939, on the other hand, are integrally formed or pre-assembledprior to anchorage of the fixture in the femur neck by screwing thefixture into the femur neck laterally through the resected section leftafter resection of the femur head.

In the femur fixtures disclosed in WO93/01769, WO93/16663 andWO97/25939, the first cylindrical section steps into the secondcylindrical section. A drawback of this type of implant is the amount ofstress present between the bone and the implant following implantation.

SUMMARY OF THE INVENTION

Thus, the object of the present invention is to provide an implant wherethe above mentioned drawback is reduced.

This and other objects are achieved according to the present inventionby providing an implant having the features defined in the independentclaim. Preferred embodiments are defined in the dependent claims.

According to the present invention there is provided an implant forinsertion into and permanent anchorage in bone tissue, comprising anintraosseous anchoring structure of a generally circular cross-section,said anchoring structure comprising a first cylindrical section of afirst diameter and a second cylindrical section of a second diameter,said second diameter being less than said first diameter, said first andsecond cylindrical sections each being provided with a screw threadprofile. The implant is characterised in that said anchoring structurecomprises a tapered connecting section provided between andinterconnecting said first and second cylindrical sections.

The implant of the invention may be an orthopaedic implant, for examplea femur fixture for a hip-joint prosthesis as in the embodimenthereinafter to be described.

Thus, the present invention is based on the advantageous idea ofproviding an implant of the above-mentioned type with a taperedconnecting section between the first and second screw-threaded,cylindrical sections.

The provision of a tapered connecting section would overcome thedrawback mentioned above and provide a number of additional advantages.First, the stresses induced by the sharp, step-wise transition presentin the prior art implants between the cylindrical sections of differingdiameters is radically reduced using the tapered connecting section ofthe present invention.

A further advantage resulting from the provision of a tapered connectingsection when the implant is used for implantation in a cavity ofcorresponding shape formed in bone tissue, is that the insertion of theimplant is facilitated. This is because the distal end of the implantcan be guided by the tapered section of the cavity which is arranged forinteracting with the tapered connecting section. Thus, the cavity willhave no surface facing directly opposite the insertion direction of theimplant, as is the case with the prior art implants (see item 50 of FIG.5 in WO 97/25939).

A still further advantage is that the provision of a tapered connectingsection provides a wedging effect during implantation of the implantinto bone tissue. This wedging effect improves the short and long termstability of the implant following said implantation. This is mainly dueto the radial force component of the normal contact force between theimplant and the cancellous bone tissue that surrounds the connectingsection upon implantation.

If the longitudinal force exerted by the surrounding tissue on theimplant of the invention and on the above-mentioned prior art implants,following implantation of the implant, is essentially the same, then thetransversal force on the implant of the invention will be greatercompared to the prior art stepped implants. This is because thelongitudinal force is carried by the inclined surface of the taperedconnecting section, whereby the resulting transversal force will have aradial force component. This radial force component is not present inthe prior art implants since only the radial end portion (e.g. item 10of FIG. 1 in WO 97/25939) of the proximal cylindrical section (e.g. item2 of FIG. 1 in WO 97/25939) carries the corresponding longitudinalforce.

While the overall transversal force is increased, the actual force persurface unit is not necessarily increased. This is due to the fact thatthe contact surface carrying the longitudinal contact forces will beconsiderably greater with the tapered connecting section of the presentinvention as is the case with said radial end portion of the prior artimplants.

Also, the provision of the inventive tapered connecting section improvesthe integration between the implant and the cancellous bone tissue(termed “osseointegration” in the art). This is mainly due to the shapeof the contact surface of the connecting section, but also to theincreased overall contact force exerted on the implant by the cancellousbone tissue.

Ordinarily, the first cylindrical section is disposed proximally of thesecond cylindrical section with the taper of the connecting sectioninclining inwardly in the distal direction, as in the embodimenthereinafter to be described.

Preferably, the tapered connecting section has a frusto-conical profile,even though other tapering shapes are conceivable without departing fromthe scope of the present invention.

According to preferred embodiments of the invention, the diameter of thefirst cylindrical section is adapted to the actual size and shape of thefemur of the particular patient for whom the implant is intended. Thus,the diameter of the first cylindrical section can vary considerably.However, the diameter of the second cylindrical portion is preferablydimensioned to be within a short, limited range. Thus, the flank angleof the connecting section may vary in dependence of the actualdimensions of the first and second cylindrical sections. Preferably, theflank angle can be varied in the range of 10°-50°, and more preferablyin the range of 200-400.

Furthermore, the longitudinal extension of the connecting section ispreferably in the range of 5-15 mm, preferably in the range of 7-11 mm.

Advantageously, the end of the tapered connecting section interfacingthe first cylindrical section has essentially the same diameter as thefirst cylindrical section. Likewise, the end of the tapered connectingsection interfacing the second cylindrical section advantageously hasessentially the same diameter as the second cylindrical section.

According to preferred embodiments of the invention, the connectingsection is at least partly provided with a roughened surface. This wouldeven further promote the osseointegration process at the transition areabetween the cylindrical sections. The roughened surface could beachieved through blasting, preferably grit-blasting, etching, or thelike. Alternatively or additionally, a circumferentially orientedroughness, preferably machined, could be provided on the connectingsection. Such circumferentially oriented roughness could for instance beprovided in the form of grooves, beads, tracks, or screw threads.

According to preferred embodiments of the invention the taperedconnecting section is at least partly provided with a screw threadprofile. The screw thread profile of the tapered connecting section ispreferably different from the screw thread profiles on the first andsecond cylindrical sections. Preferably, the height of the screw threadprofile of the connecting section is less than the height of the screwthread profiles of the cylindrical sections. Advantageously, the screwthread profile of the connecting section are in the form of microthreadshaving a height of less than 0.3 mm, preferably in the range of 0.1-0.25mm, even more preferably about 0.2 mm.

Other differences in screw thread profiles are also conceivable, such asthe screw thread profile of the connecting section having a pitch lessthan the pitch of the screw thread profiles of the cylindrical sections.

Alternatively, the screw thread profile of the connecting section isessentially the same as that of the cylindrical sections.

Where the circumferential roughness is provided in the form of beads ortracks, the height of said circumferential roughness is preferably lessthan 0.3 mm, more preferably in the range of 0.1-0.25 mm, and even morepreferably approximately 0.2 mm.

Alternatively, at least part of the surface or the entire surface of theconnecting section may be left smooth, or even polished.

In an embodiment of the invention, such as the one hereinafter to bedescribed, one or more self-tapping cutting recesses are provided atleast in part on the tapered connecting section. Said cutting recessespreferably being equi-spaced and circumferentially arranged.

According to preferred embodiments of the invention, the implant alsocomprises a tapered proximal section interconnecting the firstcylindrical portion with a head section of the implant. The taperedproximal section provides increased contact between the implant andsurrounding cortical bone tissue and improves the stability of theimplant when anchored in bone tissue.

The tapered proximal section advantageously has a frusto-conical shapewith a flank angle that is preferably in the range of 8°-150, and evenmore preferably in the range of 100-130, and even more preferablyapproximately 12°. Preferably, the end of the tapered proximal sectionabutting the first cylindrical section has a diameter essentially equalto the diameter of the first cylindrical section.

Preferably, the surface of the tapered proximal section is provided witha circumferentially oriented roughness, for instance in the form ofcircumferential beads or screw threads. The height of the beads or screwthreads is preferably no greater than 0.3 mm, more preferably in therange of 0.1-0.25 mm, and even more preferably approximately 0.2 mm.

Preferably, said beads or screw threads engage with the cortex 34 of thefemur neck at the resected surface. As a result, a stronger short termanchorage of the implant is provided. Also, a stronger long termanchorage is provided due to the improved osseointegration between thetapered proximal section and the surrounding bone tissue.

According to embodiments of the present invention, the above-mentionedhead section is provided with a collar abutting the tapered proximalsection, which collar delimits the insertion of the implant into bonetissue. Preferably, the surface of the collar facing the proximalsection is inclined inwardly so as to mate with a resected bone tissuesurface that has been given a correspondingly inclined shape.Preferably, the angle of inclination is within the range of 100-200,preferably approximately 15°. Alternatively, the surface of the collarfacing the proximal section is given a concave shape, so as to mate witha convex bone tissue surface. Thereby, an improved contact between theimplant and the bone surface can be obtained.

Preferably, said collar surface is provided with radially spacedcircular beads or grooves for increasing the stability of the insertedimplant and promote the osseointegration between the implant and thebone tissue. Preferably, these beads have a height in the range of0.1-0.5 mm, preferably in the range of 0.2-0.4 mm, and even morepreferably approximately 0.3 mm.

By way of example, an embodiment of the invention will now be describedwith reference to the accompanying Figures of drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a femur fixture for a hip-jointprosthesis in accordance with an embodiment of the invention,

FIG. 2 is an opposite perspective view of the femur fixture shown inFIG. 1,

FIG. 3 is a longitudinal side view of the femur fixture,

FIG. 4 is a longitudinal sectional view of the femur fixture,

FIG. 5 is an enlarged fragmentary sectional view showing the taperedproximal section and the head of the femur fixture,

FIG. 6 is a bottom view of the femur fixture,

FIG. 7 is a fragmentary sectional view of the collum of the human femur,with a cavity formed therein for reception of the femur fixture,

FIG. 8 is a fragmentary sectional view of the collum of the human femur,with the femur fixture inserted therein,

FIG. 9 is a perspective view similar to FIG. 2 of the femur fixture witha roughened surface which is at least partially blasted;

FIG. 10 is a perspective view similar to FIG. 9 of the femur fixturewith a connecting section having circumferential beads;

FIG. 11 is a longitudinal side view similar to FIG. 3 of the femurfixture with a connecting section having a screw thread profile;

FIG. 12 is a longitudinal sectional view similar to FIG. 4 of the femurfixture with a connecting section having the screw profile;

FIG. 13 is a longitudinal side view similar to FIG. 11 of the femurfixture with a connecting section have circumferential beads; and

FIG. 14 is a perspective view similar to FIG. 10 of the femur fixturewith a connecting section having a screw thread.

DESCRIPTION OF EXEMPLARY EMBODIMENT

With reference to FIGS. 1-8, there is shown an integrally formed femurfixture 1 for a hip-joint prosthesis preferably made from commerciallypure titanium and consisting of (i) an intraosseous anchoring section 3of circular cross-section, and (ii) a head section S. The anchoringsection 3 is intended for insertion laterally into a cavity 30 ofcomplementary profile (FIG. 7), said cavity 30 being drilled into theneck of a femur through a resected section 33 made by resection of thehead of the femur. The head section 5 of the fixture, which willprotrude from the resected section 33 when the intraosseous anchoringsection 3 is located in the cavity 30 (FIG. 8), is arranged forsupporting a ball 25 of the hip-joint prosthesis which interacts withthe anatomical acetabular cavity or an acetabular part of the hip-jointprosthesis where a total hip-joint prosthesis is required.

As can be seen in FIGS. 1-3, the intraosseous anchoring section 3 hasproximal and distal cylindrical sections 11, 13 of different outerdiameter, with the diameter of the proximal cylindrical section 11 beinggreater than that of the distal cylindrical section 13. The intraosseousanchoring section 3 further has a tapered terminal distal section 12,contiguous with the distal cylindrical section 13, a frusto-conicalconnecting section 15 connecting the proximal cylindrical section 11 tothe distal cylindrical section 13, and a frusto-conical proximal section18 connecting the proximal cylindrical section 11 to the head section 5.

The proximal cylindrical section 11 presents a screw-threaded outersurface for screwing into an outer bone cavity section 32 of saidcavity. The distal cylindrical section 13 also presents a screw-threadedouter surface, for screwing into a narrow drilled hole 31, which iscoaxial with said outer cavity section 32. The screw-threads of theproximal cylindrical section 11 have the same pitch and height as thoseof the distal cylindrical section 13.

The major diameters of the screw threads on the proximal and distalcylindrical sections 11, 13 are sized to be greater than the innerdiameter of complementary cylindrical sections of the outer cavitysection 32 and the drilled hole 31 provided in the cavity 30 of thefemur neck (See FIG. 7). Accordingly, the intraosseous anchoring section3 is able to be anchored in the cavity 30 by screwing of the femurfixture 1 into the cavity 30, with the screw threads on the proximal anddistal cylindrical sections 11, 13 threading into the bone tissue in theboundary wall of the cavity 30.

As seen in FIG. 8, the diameter of the proximal cylindrical section 11is in fact sized such that the threads thereon register in theperipheral layer of cortical bone 34 in the femur neck, as outlined inWO93/16663 and WO97/25939. The threads on the proximal cylindricalsection 11 are thus secured in the stronger cortical bone 34 as opposedto the spongier cancellous bone 35, thereby giving the femur fixture 1greater fixation in the femur neck. Due to the fact that the femurdimensions can vary from patient to patient, the diameter of theproximal cylindrical section can vary in the range from approximately16-26 mm (cf. FIGS. 3 and 8).

As illustrated in FIG. 8, the axial length of the intraosseous anchoringsection 3 is such that in the anchored position of the intraosseousanchoring section 3, the distal end 12 thereof projects through thelateral cortex 34 of the femur.

With reference to FIGS. 3-5, the frusto-conical proximal section 18 alsohas threads thereon. The height of these threads is 0.2 mm (so-calledmicrothreads) which is less than that of the threads on the proximal anddistal cylindrical sections 11, 13. Further, the frusto-conical proximalsection 18 is sized so that the microthreads engage with the cortex 34of the femur neck at the resected surface. In the embodiment describedherein, the frusto-conical terminal proximal section 18 has a flankangle of approximately 12°, and an axial extent of approximately 8 mm.

The distal diameter of the proximal section 18 is adapted to thediameter of the neighbouring proximal cylindrical section 11, such thatthere are no sharp edges in the transition area between thefrusto-conical proximal section 18 and the proximal cylindrical section11. Consequently, the proximal diameter of the frusto-conical proximalsection 18 is in the range of approximately 20-30 mm.

The diameter of the distal cylindrical section 13 does not have to bevaried in dependence of the femur dimensions of the patient. Thediameter of the distal cylindrical section 13 is approximately 11 mm, orwithin the range of 10-12 mm.

The frusto-conical connecting section 15 interconnects the proximal anddistal cylindrical sections 11, 13 to one another. In this embodiment,the diameters at the respective end of the connecting section 15correspond to the diameters of the proximal and distal cylindricalsections 11, 13, respectively. In other words, the distal end of theconnecting section 15 has essentially the same diameter as the distalcylindrical section 13, and the proximal end of the connecting section15 has essentially the same diameter as the proximal cylindrical section11.

As a result of the fact that the diameter of the proximal cylindricalsection 11 can be varied between different femur fixtures, while thediameter of the distal cylindrical section 13 is not varied, thedimensions of the connecting section will be varied in accordance withthe varying difference in diameter between the proximal cylindricalsection 11 and the distal cylindrical section 13. Since the axial extentof the connecting section is kept relatively short, i.e. within therange of approximately 7.5-10.5 mm, the flank angle of the connectingsection can vary from approximately 200 for the narrowest fixturealternative, up to approximately 370 for the widest fixture alternative.

In the herein described embodiment of the invention, the surface of thefrusto-conical connecting section 15 is provided with a grit-blastedsurface for promoting the osseointegration between the surface and thesurrounding cancellous bone tissue. The surface could also, oralternatively, be provided with a screw thread profile for promotingsaid osseointegration and improve the anchorage of the femur fixture 1.As a further alternative, the frusto-conical connecting section 15 maybe left smooth, even polished.

As can be seen in FIGS. 2 and 3, bridging the boundary between theproximal cylindrical section 11 and the frusto-conical connectingsection 15 are a series of equi-spaced, circumferentially-arranged,sharp-edged cutting recesses or notches 14 for self-tapping into aprecut outer bone cavity section 32. The cutting recesses 14 eachcommunicate with a channel 16 in the proximal cylindrical section 11 forautologous transplantation of the bone cut by the cutting recesses 14 asthe femur fixture 1 is screwed into the bore in the femur neck, asdetailed in WO97/25939.

Further, bridging the boundary between the distal cylindrical section 13and the tapered terminal distal section 12 are also a series of short,sharp-edged circumferentially-arranged cutting recesses 17 for thedistal cylindrical section 13 to be self-tapped into said drilled,relatively narrow hole 31.

With reference to FIGS. 1, 7 and 8, the head section of the femurfixture 1 has a collar section 20 and a tapered mounting section 23 forthe ball component 25 of the hip-joint prosthesis to be mounted on. Themounting section 23 is provided with a recess 24 for reception of theball component 25. The collar section 20 delimits the insertion of theintraosseous anchoring section 3 into the bore in the femur neck byabutting with the resected femur section 33 adjacent the opening to thecavity 30. As can be seen in FIG. 5, the distal surface 21 (FIG. 5) isinclined inwardly for mating with a correspondingly inclined bonesurface the resected femur section 33 (FIG. 7). The angle of inclinationin the embodiment herein described is approximately 15°. Further, asseen in FIG. 6, for improved anchorage and osseointegration, the distalsurface 21 of the collar section 20 is provided with radially spaced,circumferential beads 22, said beads having a height of approximately0.3 mm.

The surgical procedures described in WO93/16663 and WO97/25939 forimplanting the femur fixtures disclosed therein can also be adapted forimplantation of the femur fixture 1 and as such are incorporated hereinby reference.

The anchorage of the femur fixture 1 is primarily reliant on theregistration of the threads in the bone of the femur, principally theregistration of the threads on the proximal cylindrical section 11 inthe cortex 34 of the femur neck and the registration of the threads onthe distal cylindrical section 13 in the lateral cortex 34 of the femur.This is in distinction to femur fixtures which rely on a thrust platemechanism for their fixation, for example as in GB-A-2033755.

The femur fixture 1 herein described with reference to the accompanyingfigures can be varied in numerous ways within the scope of theinvention. For instance, the femur fixture 1 could be in the form of anassembly in which the component parts are assembled (i) for insertionthereof laterally into the bore as a one-piece structure, as disclosedin WO93/16663, or (ii) by connecting the parts together in the bore, asdisclosed in WO93/01769. The femur fixture 1 could also be made from anybiocompatible patible material of strength sufficient to withstand theloads imposed upon it in situ.

Turning now to FIG. 9 and embodiment of the femur fixture is shown witha connecting section 15 a having a roughened surface which is at leastpartly a blasted surface. In FIG. 10, a connecting section 15 b is shownhaving a roughened surface 15 which is at least partly provided with acircumferentially oriented roughness in the form of circumferentialbeads that have a height less than that of the screw thread profiles ofthe first and second cylindrical sections and no greater than 0.3 mm.

In FIGS. 11 and 12, a connecting section 15 c is shown having aroughened surface which is at least partly provided with acircumferentially orientated roughness in the shape of a screw threadprofile that has a height less than that of the screw thread profiles ofthe first and second cylindrical sections and no greater than 0.3 mm.FIG. 13 illustrates a proximal section which is provided with aroughness 18 a in the form of circumferential beads. In FIG. 14, aconnecting section 15 d is illustrated having a roughened surface whichis at least partly provided with a circumferentially oriented roughnessin the shape of a screw thread profile that has a height essentially thesame as that of the screw thread profiles of the first and secondcylindrical sections.

It will be appreciated that the invention has been described withreference to an exemplary embodiment and that the invention can bevaried in many different ways within the scope of the appended claims.For instance, the implant is not confined to use as a femur fixture fora hip-joint prosthesis. As an example, the implant could take the formof a bone fixation screw. It will further be appreciated that the use inthe appended claims of reference numerals from the Figures of drawingsis for the purposes of illustration and not to be construed as having alimiting effect on the claims.

What is claimed is:
 1. An orthopaedic fixture for an orthopaedicprosthesis, said fixture being arranged to be screwed into a bore havingan open end in a resected bone surface and arranged to be permanentlyanchored in bone tissue, comprising a ball carrying portion and anintraosseous anchoring structure of a generally circular cross-section,said anchoring structure comprising a first cylindrical section of afirst diameter and a second cylindrical section of a second diameter,said second diameter being less than said first diameter, said first andsecond cylindrical sections each being provided with a screw threadprofile of the same hand, characterized in that said anchoring structurecomprises a tapered connecting section provided between andinterconnecting said first and second cylindrical sections, wherein thediameters at the respective end of the connecting section correspond tothe diameters of the first and the second cylindrical sections,respectively, and in that said fixture comprises a collar section whichis arranged proximally to said first cylindrical section of theanchoring structure adapted to abut said resected bone surface.
 2. Afixture as claimed in claim 1, wherein the implant is a femur fixture ofa hip-joint prosthesis.
 3. A fixture as claimed in claim 1, wherein saidconnecting section has a frusto-conical shape.
 4. A fixture as claimedin claim 3, wherein said connecting section at one end has a basediameter essentially equal to said first diameter of said firstcylindrical section, and at the other end has a top diameter essentiallyequal to said second diameter of said second cylindrical section.
 5. Afixture as claimed in claim 3, wherein said connecting section has aflank angle in the range of 10°-50°.
 6. A fixture as claimed in claim 1,wherein said connecting section is at least partly provided with aroughened surface.
 7. A fixture as claimed in claim 6, wherein saidroughened surface is at least partly a blasted surface.
 8. A fixture asclaimed in claim 6, wherein said roughened surface is at least partlyprovided with a circumferentially oriented roughness.
 9. A fixture asclaimed in claim 8, wherein said circumferentially oriented roughness isin the form of circumferential beads.
 10. A fixture as claimed in claim9 wherein said circumferential beads has a height less than that of thescrew thread profiles of said first and second cylindrical sections. 11.A fixture as claimed in claim 10, wherein the height of saidcircumferential beads is no greater than 0.3 mm.
 12. A fixture asclaimed in claim 8, wherein said circumferentially oriented roughness isin the shape of a screw thread profile.
 13. A fixture as claimed inclaim 12, wherein the screw thread profile of said connecting sectiondiffers from the screw thread profiles of said first and secondcylindrical sections.
 14. A fixture as claimed in claim 13, wherein thescrew thread profile of said connecting section has a height less thanthat of the screw thread profile of said first and second cylindricalsections.
 15. A fixture as claimed in claim 14, wherein the screw threadprofile of said connecting section is in the form of microthreads.
 16. Afixture as claimed in claim 15, wherein the height of said microthreadsis no greater than 0.3 mm.
 17. A fixture as claimed in claim 12, whereinthe heights of the screw thread profiles of said first and secondcylindrical sections and said connecting section are essentially thesame.
 18. A fixture as claimed in claim 1, wherein said connectingsection is at least partly provided with a smooth surface.
 19. A fixtureas claimed in claim 1, wherein the entire surface of said connectingsection is smooth.
 20. A fixture as claimed in claim 1, wherein one ormore self-tapping cutting recesses are provided at least in part on saidconnecting section.
 21. A fixture as claimed in claim 1, wherein saidimplant comprises a head section, and wherein said anchoring structurecomprises a tapered proximal section being provided between andinterconnecting said first cylindrical section and said head section.22. A fixture as claimed in claim 21, wherein said proximal section hasa frustro-conical shape.
 23. A fixture as claimed in claim 22, whereinsaid proximal section at the end interfacing said first cylindricalsection has a diameter essentially equal to said first diameter of saidfirst cylindrical section.
 24. A fixture as claimed in claim 22, whereinsaid proximal section has a flank angle in the range of 8°-15°.
 25. Afixture as claimed in claim 21, wherein said proximal section is atleast partly provided with a circumferentially oriented roughness.
 26. Afixture as claimed in claim 25, wherein said circumferentially orientedroughness is in the form of circumferential beads.
 27. A fixture asclaimed in claim 25, wherein said circumferentially oriented roughnessis in the form of a screw thread profile.
 28. A fixture as claimed inclaim 26, wherein the height of said circumferentially orientedroughness is no greater than 0.3 mm.
 29. A fixture as claimed in claim21, wherein said collar section forms part of the head section.
 30. Afixture as claimed in claim 29, wherein said distal surface is inclinedinwardly towards the body of the collar section.
 31. A fixture asclaimed in claim 30, wherein said distal surface is inclined inwardly atan inclination angle within the range of 10°-20°.
 32. A fixture asclaimed in claim 29, wherein said distal surface is concave.
 33. Afixture as claimed in claim 29, wherein said distal surface is providedwith radially spaced circular beads.
 34. A fixture as claimed in claim33, wherein said circular beads have a height in the range of 0.1-0.5mm.